A method and apparatus is disclosed for generating a clean, low Btu fuel gas by the reaction of a carbonaceous fuel limestone mixture with oxygen and steam in a downdraft fixed bed gasifier. The product gas and slag are discharged from the lower end of the gasifier after having passed through an incandescent char matrix in which contaminants such as sulfur are scrubbed from the gas. A portion of the hot product gas is withdrawn from the upper end of the gasifier to dry, preheat, and devolatilize freshly introduced fuel material. This gas which contains a heavy oily emulsion of tars and sulfur compounds is then reintroduced into the gasifier. The gasifier is capable of gasifying essentially any hydrocarbon material such as, refuse, lignite, anthracite, bituminous coal, coke, oil, liquid waste and many others, including manure and other farm waste.